PHP Iterables

PHP - What is an Iterable?

An iterable is any value which can be looped through with a foreach() loop.

The iterable pseudo-type was introduced in PHP 7.1, and it can be used as a data type for function arguments and function return values.

PHP - Using Iterables

The iterable keyword can be used as a data type of a function argument or as the return type of a function:

Use an iterable function argument:

< ?php
function printIterable(iterable $myIterable) {
  foreach($myIterable as $item) {
    echo $item;
  }
}

$arr = ["a", "b", "c"];
printIterable($arr);
?>

Return an iterable:

< ?php
function getIterable():iterable {
  return ["a", "b", "c"];
}

$myIterable = getIterable();
foreach($myIterable as $item) {
  echo $item;
}
?>

PHP - Creating Iterables

All arrays are iterables, so any array can be used as an argument of a function that requires an iterable.

Any object that implements the Iterator interface can be used as an argument of a function that requires an iterable.

An iterator contains a list of items and provides methods to loop through them. It keeps a pointer to one of the elements in the list. Each item in the list should have a key which can be used to find the item.

An iterator must have these methods:

• current() - Returns the element that the pointer is currently pointing to. It can be any data type
• key() Returns the key associated with the current element in the list. It can only be an integer, float, boolean or string
• next() Moves the pointer to the next element in the list
• rewind() Moves the pointer to the first element in the list
• valid() If the internal pointer is not pointing to any element (for example, if next() was called at the end of the list), this should return false. It returns true in any other case

Implement the Iterator interface and use it as an iterable:

< ?php
// Create an Iterator
class MyIterator implements Iterator {
  private $items = [];
  private $pointer = 0;

  public function __construct($items) {
    // array_values() makes sure that the keys are numbers
    $this->items = array_values($items);
  }

  public function current() {
    return $this->items[$this->pointer];
  }

  public function key() {
    return $this->pointer;
  }

  public function next() {
    $this->pointer++;
  }

  public function rewind() {
    $this->pointer = 0;
  }

  public function valid() {
    // count() indicates how many items are in the list
    return $this->pointer < count($this->items);
  }
}

// A function that uses iterables
function printIterable(iterable $myIterable) {
  foreach($myIterable as $item) {
    echo $item;
  }
}

// Use the iterator as an iterable
$iterator = new MyIterator(["a", "b", "c"]);
printIterable($iterator);
?>